Harmonic reduction circuits



y 1933- I r o. E. KEALL 2,115,858

HARMONIC REDUCTION CIRCUITS Filed Oct. 5, 1955 -|NVENTOR ash 01.05. 1664M ATTORNEY Patented May 3, 193

PATENT OFFICE 2,115,858 HARMONIC REDUCTION cmcorrs Oswold Edward Keall, Chelmsford, England, assignor to Radio Corporation of America, a corporation of Delaware Application October 5, 1935, Serial No. 43,687 In Great Britain October 10, 1934 2 Claims. (Cl. 250-36) This invention relates to thermionic valve circuit arrangements, and more particularly to thermionic valve amplifier, oscillator, and like arrangements of the kind embodying thermionic valves having non-rectilinear characteristics, i. e.,

valves whose input voltage-output current characteristics are not rectilinear.

It is well known that in a thermionic amplifier, or oscillator, incorporating a valve, or valves (for example of the tetrode type), having nonrectilinear characteristics, one result of the nonrectilinearity of characteristic is the production in the output circuit of energy which is at one,

or more, harmonics of the fundamental frequency to be amplified, or generated. For many purposes such production of harmonic frequency energy is undesired, and various proposals have been made to avoid, or reduce, such harmonic frequency output energy. In general, however, known proposals for this purpose involve reduction of the fundamental frequency output energy, and/or are complex and difficult to adjust.

The object of the present invention is to provide an'improved thermionic valve circuit arrangement embodying a valve, or valves, having non-rectilinear characteristics, and which shall not sufier from the disadvantages above referred to.

The object of the present invention is achieved by back-coupling a thermionic valve, or valves,

of non-rectilinear characteristic by a circuit arrangement including a portion resonant to the fundamental frequency, and such that the output energy at fundamental frequency is left substantially undiminished by reason of the back-coupling whereby harmonic frequency energy output is reduced by reason of the back-coupling. In the drawing: Fig. 1 shows the invention applied to an amplifier,

Fig. 2 shows a modification, embodied in a regenerative amplifier or oscillator,

Fig. 3 illustrates a modification of Fig. 2. In one way ofcarrying out this invention, as applied to a thermionic amplifier of the tetrode type, a four electrode screenedgrid valve l (for example a valve of the so-called variable mu type) has its anode 2 connected through the usual output impedance 3, in series with a source 5 4 of anode potential, to a point 5 which may be earthed. The point 5 is in turn connected to the cathode 6 through a resistance 1 shunted by a series tuned, or acceptor, circuit consisting of inductance 8 and capacity 9 in series and reso- 55 nant to the fundamental frequency to be ampli- The point 5 which, as stated, may be earthed as shown, is connected to the control, or inner, grid ID of the tetrode through an input impedance ii across which the fundamental frequency to be amplified is applied, and, if desired, a bias battery ll may be inserted in this connection to the control grid, for example, between the input impedance II and the point 5. "The screen grid I2 is connected to a suitable source of positive bias potential, for example, as shown, 10 to a point l3 upon the anode battery, as in, the usual way, and amplified output voltages are taken off across the output impedance 3.

It will be appreciated ,that with this circuit arrangement anode current will pass through 15 the resistance 1 in the cathode lead, and accordingly the control grid ill will, in general, receive negative bias due to the passage of such anode current, which negative bias may be increased or decreased, if desired, by the provision of the bias battery in the control grid circuit. The cathode network comprising the resistance 1 in the cathode lead, and the series tuned circuit 8, 9, in shunt therewith, is common to both grid and anode circuits of the valve.

Due to its position in the anode circuitits impedance at any particular frequency will determine the alternating current voltage developed across it. At the fundamental frequency this voltage will be substantially zero, but at fre- 30 quencies higher than the fundamental frequency the series resonant, or acceptor, circuit is, of course, inductive in character. By proper choice of the inductance and capacity values in this resonant circuit its impedance to the second, or 35 higher, harmonics of the fundamental frequency may be made large as compared with the resistance in the cathode lead so that for harmonics of the fundamental frequency the effective value of the cathode network is that of the resistance 40 itself, the shunt acceptor circuit presenting at such harmonic frequencies, an impedance which is high as compared with the cathode resistance. The cathode network will, therefore, present a substantially constant impedance to harmonic 45 frequencies produced due to non-rectilinearity of valve characteristic.

Due to its position in the grid circuit voltages developed across the cathode network by second, or higher, harmonics of the fundamental frequency will give rise to variations of anode current at these frequencies, but these variations will be out of phase with those produced in the firstplace by the fundamental frequency alone. There will, therefore, be a reduction of the harmonic energy in the output circuit. Since, however, the impedance of the cathode network is substantially zero at the fundamental frequency, and, therefore, substantially no voltage at this frequency will be developed across the network, the amplitude of the fundamental frequency in the output circuit will be substantially undiminished.

In the modification shown in Fig. 2 a triode la is employed, the grid llla being connected to the earthed point 5, and to the negative terminal of the source 4 of anode potential through a parallel tuned circuit lla, llb tuned to the fundamental frequency. The point 5 is connected to the oathode 6a through a resistance 1 shunted by a series resonant circuit 8, 9, also tuned to the fundamental frequency. The positive terminal of the source 4 of anode voltage is connected to the anode 2a through a further parallel tuned circuit 3a--3b resonant to the fundamental frequency, and the coil 3a in this parallel tuned circuit is magnetically back coupled to the coil lla in the parallel tuned circuit Ila-l lb. The embodiment now being described can be used either as an amplifier, or as an oscillator, in dependence upon the amount of back-coupling, for if the back-coupling is insufficient for oscillation generation, the parallel tuned circuit llallb may be used as an input circuit for the frequency to be amplified, the circuit 3a3b constituting the output circuit. It will be appreciated that the resistance I gives a negative feed back at all frequencies except the fundamental frequency, for at this frequency the said resistance is effectively short-circuited by the series resonant, or acceptor, circuit 8, 9.

If it is desired to provide for adjustment of grid bias the circuit may be modified, as shown in Fig. 3, by inserting a blocking condenser ll between the grid Ida and the adjacent end of the parallel tuned circuit Hal lb, and connecting said grid through a resistance l5 to an adjustable tapping point l6 upon the resistance 1 in the cathode lead.

The invention is of wide application. It may be applied to thermionic amplifiers generally and to thermionic oscillators generally, and is of particular advantage in the case of oscillators since it may be applied thereto to improve the wave form thereof by the reduction of harmonics. The invention is, also, of considerable advantage as applied to radio receivers, and in particular to radio receivers of the heterodyne type, for, by applying the said invention to the local oscillators of such receivers, disturbing whistles" due to heterodyne effects between oscillator harmonics and input signals, may be reduced. This advantage is found particularly noticeable in the case of superheterodyne receivers having aperiodic radio frequency input circuits, for in such receivers whistles from the above mentioned cause are very prone to occur and constitute/ a serious operating disadvantage.

What is claimed is:

1. In an electrical oscillator network of the type comprising an electron discharge tube provided with at least a cathode, anode and-input electrode, a wave input circuit connected between the cathode and input electrode, said input cir-' cuit being constructed to be tuned at all times to a desired fundamental frequency, a wave output circuit connected between the cathode and anode, a resistor in the space current path of the tube and common to the input and output circuits, said input electrode and cathode being connected to points on the resistor such that the input electrode is biased negatively by the direct current voltage developed across the resistor by the space current, a reactive path in shunt with said resistor and being series resonant to said fundamental frequency, the constants of the reactive path being such that its impedance to harmonics of the fundamental frequency is sufficiently higher than the resistance of said resistor to cause the harmonic current to flow solely through the resistor and harmonic voltage developed across the resistor being impressed on said input electrodes in degenerative phase, means for reactively coupling said two circuits in a regenerative sense to produce oscillations in said network of said fundamental frequency.

2. An oscillator network comprising a tube between whose input electrodes is connected a resonant circuit fixedly tuned to a desired frequency,

and between whose output electrodes is connected a resonant circuit fixedly tuned to the same frequency, said circuits being reactively coupled in a regenerative sense, a resistor in the space current path of the tube connected to provide a biasing potential difference between the input electrodes, and means in shunt with said resistor act.- ing to short circuit the latter for currents of said frequency, but acting as an open circuit for currents of harmonic frequency and said input electrodes being connected to said resistor so that harmonic voltage developed across the latter is impressed in degenerative phase between the former.

OSWOLD EDWARD KEALL. 

